Even if math isn't your favorite subject, there are some numbers that we just can't miscalculate. Sun protection factor (SPF) is one of them and is extremely important for our skin, and should be at the forefront of our Summer health practices. Here are some essential sunscreen facts to keep in mind while you bathe:

Believe it or not, SPFs don't add up like a simple math problem. For example, if you apply SPF 10 and SPF 30, you won't magically have SPF 40. Sunscreens don't work this way because their active ingredients do not add on top of one another. However, using two SPFs would increase your overall coverage and lessen the chance that you missed a spot while applying.

It's weird, but different sunscreens also have different levels of protection against UVB rays, which means that they don't increase proportionally as some people believe. Just because you bump up your protection from SPF 15 to SPF 30 doesn't mean you'll be rewarded with double protection. A higher SPF does provides greater protection from those damaging short waves, so think big!

SPF makeup can dilute the SPF in your moisturizer and actually decrease your skin's protection factor. So if you're going to use an SPF moisturizer first, then make sure to apply makeup without SPF in order to prevent reduction of your sun protection.

Discover the other key sunscreen rules to follow when you read more.

Sunscreen isn't just found in a bottle. You should make sure to bring other items — sunglasses, cover-ups, umbrellas, wide-brimmed hats, etc. — when you sunbathe because protective clothing will help to avoid getting too much sun and could come in handy when you've had enough.

Since no sunscreen is waterproof, applying your SPF throughout the day is crucial. Even if a product passes FDA tests to claim "water resistance," this usually only lasts for about 40 to 80 minutes.